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Sampling and Measurement for Volatile Organic Compounds PowerPoint PPT Presentation


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Sampling and Measurement for Volatile Organic Compounds. Types of analytical instruments. Personal Monitors Light weight monitors Can be conveniently carried or worn by a person Portable monitors Can be moved during sampling Stationary monitors Have to be operated from a fixed location.

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Sampling and Measurement for Volatile Organic Compounds

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Sampling and measurement for volatile organic compounds l.jpg

Sampling and Measurement for Volatile Organic Compounds


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Types of analytical instruments

  • Personal Monitors

    • Light weight monitors

    • Can be conveniently carried or worn by a person

  • Portable monitors

    • Can be moved during sampling

  • Stationary monitors

    • Have to be operated from a fixed location

Volatile Organic Compounds


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Types of Sampling

  • Active Sampling

    • Contaminated air is directly drawn into a sensor or collector

    • Provides a real time analysis of the sample

  • Passive Sampling

    • Air diffuses into the collector

    • Longer period of time must be allowed for accurate results

Volatile Organic Compounds


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Sampling methods

  • Principles of Sampling Collectors

    1.Air displacement

    2.Condensation

    3.Gas washing or absorption

    4.Adsorption

Volatile Organic Compounds


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Air Displacement Sampling Method

  • Evacuated flasks or plastic bags are used

  • Samples are collected by opening the inlet to the contaminant-laden air

  • Sample is subsequently analyzed using a suitable analytical method

Volatile Organic Compounds


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Condensation Sampling Method

  • Air is passed through a U-tube or a suitable container

  • Sample is subsequently cooled to below the boiling point of the pollutant

  • In most cases a liquid nitrogen cryogenic trap is used.

  • Condensed liquid is eventually analyzed

Volatile Organic Compounds


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Gas washers/ Absorbers Sampling Method

  • Air containing the pollutants is bubbled through a liquid contained in an impinger

  • An aliquot of the liquid is analyzed to determine the characteristics and concentrations of the pollutants

  • Distilled water is commonly used for readily soluble gases

  • A liquid medium that reacts with the pollutants should be avoided

Volatile Organic Compounds


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Gas washers/ Absorbers Sampling Method

  • This method is suitable only for a few specific pollutants such as Formaldehyde, Phenol and Ethylene Oxide

  • Collection Efficiency can be increased by:

    • Decreasing the flow rate

    • Improving the distribution of gas phase in the liquid medium

    • Increasing the residence time

    • Using two or more collectors in series

Volatile Organic Compounds


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Adsorption Sampling Method

  • Used for measurement of emissions from materials.

  • Emissions are measured either in an environmental chamber or through “head space” analysis

  • Environmental Chamber Studies:

    • Material is placed in a chamber

    • Environmental parameters of the chamber (temperature, humidity, air exchange rates) are controlled precisely

    • Air samples are collected in solid adsorbent tubes for later analysis with a gas chromatograph

Volatile Organic Compounds


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Adsorption Sampling Method

  • “Head-Space” analysis

    • Small samples of the material are placed in a container

    • Head space gases are collected and analyzed by a gas chromatograph

    • Sampling time depends on the emission rate from the material

  • Solid adsorption media are more popular

    • Pollutants are allowed to adsorb for a predetermined period of time on a solid

    • Most frequently used solid adsorbent is activated carbon and silica gel

Volatile Organic Compounds


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Adsorption Sampling Method

  • Factors for successful use of adsorbent cartridges

    • Careful cleaning of the sorbent tube to remove background contamination

    • Accurate determination of sampling rate

  • Cleaning of cartridges

    • Heat them at a high temperature under vacuum or by flowing inert gas through the cartridge

    • Flow of inert gas should also continue during cooling

    • Another method is to wash the cartridge with a solvent and dry it at a temp. of 100-150 oC

Volatile Organic Compounds


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Analysis Methodologies

  • Colorimeters and Spectrophotometers

    • Used to quantify a specific pollutant

    • Utilize liquid reagents, chemically treated papers and glass indicating tubes

    • Chromotropic acid method

      • Most frequently used to determine the concentration of Formaldehyde

      • Measured volume of sample air is drawn through distilled water into an absorber

      • Collection efficiency greater than 95% is achieved by using two absorbers in series

Volatile Organic Compounds


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